The present invention relates generally to diesel engine systems and methods for handling diesel engine exhaust and, more particularly, to such systems and methods wherein NOx control devices can be bypassed.
Strict diesel engine emissions regulations have been proposed in the U.S. California, for example, has proposed regulations that require trucks to stop idling after no more than five minutes unless NOx emissions from the idling vehicles are limited to 30 grams/hr or less.
In response to such requirements, certain vehicles have been equipped with a “low NOx idle” feature which prevents NOx emissions over 30 grams/hr. While the particular engine operating conditions involved in a low NOx idle may vary from vehicle to vehicle, low NOx idle operating conditions typically involve operation at low load and low engine speed, such as at around 700 rpm. In addition, a low level of exhaust gas recirculation (EGR) may be employed. Other operating conditions may include that the engine be warmed up, parked, and below a particular elevation, such as 6000 feet.
During low NOx idle, exhaust temperatures are typically too low for a NOx control device such as a selective reduction catalyst (SCR) or a lean NOx trap (LNT) to operate effectively. The inventor has recognized that low NOx idle operation tends to produce low amounts of hydrocarbons which, over long term idling, can collect in the NOx control device and deactivate it and, according to an aspect of the present invention, proposes bypassing the NOx control device during low NOx idle engine operating conditions. The inventor has further recognized that the bypass system permits protection of the NOx control device when, for example, a diesel particulate filter (DPF) has an exotherm regeneration (such as may occur when exhaust gas is heated by a heater upstream of the DPF) and temperature of the exhaust gas may be too high for the NOx control device. Further, the inventor has recognized that the bypass system permits protection of the NOx control device when, due to any one or more of a variety of engine operating conditions or exhaust characteristics, it may be desirable to avoid sending the exhaust gas through the NOx control device.
In accordance with an aspect of the present invention, a diesel engine system comprises a diesel engine operable under a plurality of operating conditions, a NOx control device downstream of the engine, an exhaust line between the engine and the NOx control device, a bypass line connected to the exhaust line at a first end upstream of the NOx control device, and a bypass valve connecting the exhaust line and the bypass line, the bypass valve permitting flow through the exhaust line to the NOx control device when in an open position and preventing flow through the exhaust line to the NOx control device and permitting flow through the exhaust line to the bypass line when in a closed position. A controller is arranged to control opening and closing of the bypass valve when at least one operating condition of the plurality of operating conditions reaches a predetermined operating condition.
In accordance with another aspect of the present invention, a diesel engine system comprises a diesel engine, a NOx control device downstream of the engine, an exhaust line between the engine and the NOx control device, a bypass line connected to the exhaust line at a first end downstream of the engine and upstream of the NOx control device, and a bypass valve connecting the exhaust line and the bypass line, the bypass valve permitting flow through the exhaust line to the NOx control device when in an open position and preventing flow through the exhaust line to the NOx control device and permitting flow through the exhaust line to the bypass line when in a closed position. A controller is arranged to control opening and closing of the bypass valve when a temperature of gas in the exhaust line upstream of the NOx control device exceeds a predetermined value.
In accordance with another aspect of the present invention, a method for handling diesel engine exhaust comprises operating a diesel engine under a plurality of operating conditions, exhausting gas from the engine to an exhaust line downstream of the engine, a NOx control device being disposed in the exhaust line downstream of the engine and a bypass line being connected to the exhaust line at a first end upstream of the NOx control device, moving a bypass valve connecting the exhaust line and the bypass line between an open and a closed position, the bypass valve permitting flow through the exhaust line to the NOx control device when in the open position and preventing flow through the exhaust line to the NOx control device and permitting flow through the exhaust line to the bypass line when in the closed position, and closing the bypass valve when at least one operating condition of the plurality of operating conditions reaches a predetermined operating condition.
In accordance with yet another aspect of the present invention, a method for handling diesel engine exhaust comprises operating a diesel engine under a plurality of operating conditions, exhausting gas from the engine to an exhaust line downstream of the engine, a NOx control device being disposed in the exhaust line downstream of the engine, a bypass line being connected to the exhaust line at a first end upstream of the NOx control device, moving a bypass valve connecting the exhaust line and the bypass line between an open and a closed position, the bypass valve permitting flow through the exhaust line to the NOx control device when in the open position and preventing flow through the exhaust line to the NOx control device and permitting flow through the exhaust line to the bypass line when in the closed position, and closing the bypass valve when a temperature of gas in the exhaust line upstream of the NOx control device exceeds a predetermined value.